qemu: remove unnecessary setting of tap device online state
    
    The linkstate setting of an <interface> is only meant to change the
    online status reported to the guest system by the emulated network
    device driver in qemu, but when support for auto-creating tap devices
    for <interface type='ethernet'> was added in commit 9717d6, a chunk of
    code was also added to qemuDomainChangeNetLinkState() that sets the
    online status of the tap device (i.e. the *host* side of the
    interface) for type='ethernet'. This was never done for tap devices
    used in type='bridge' or type='network' interfaces, nor was it done in
    the past for tap devices created by external scripts for
    type='ethernet', so we shouldn't be doing it now.
    
    This patch removes the bit of code in qemuDomainChangeNetLinkState()
    that modifies online status of the tap device.